With respect to the fake hairpiece issue, i'm pretty sure it is her own hair. In the pictures of the royal family going to church on Christmas Day, Mary's hair looks a lot lighter on top then it has recently, so maybe she had it lightened a few days ago. Then when it is put up, because the colour is so fresh, the contrast between her hair is more obvious. She does seem to have quite a lot of her own natural hair. I can't imagine a hairpiece would be needed.
For the curious, the pictures from church on xmas day are over on my royals blogspot.

Actually, the Soho Velvet Wine swatch looks very similar to Mary's material. With this color, you can wear ruby jewelry (which Mary has, of course) or amethysts.

\i supposie it is diffiuclt to say becasue of photos but i thought this one was more similar, what you think?Fairvel Christmas Red Velvet
I was thinking another thing, the condecoration of the elephant must be very heavy and normally stay perfect so I guess mas be sawn to the dress, by now I am sure she knows how to walk and carry them but I am sure first time wasn't easy to keep your head up with the tiara and all the rest of the jewellery on your body you must jeep working out just to curry them!!!!!

I am almost persuaded that they are two totally different dresses. The neckline on the original was designed to carry the ruby necklace and this one wouldn't fit a choker let alone a piece of serious jewellery.

Mary doesn't seem to be a person who would have 2 dresses that are the same with slight changes made. Same color, same fabric and same cut with just a belt added and different neckline therefore I personally think its just changed from neckline to waist. The aqua dress worn in Australia recently was the same dress worn to Victoria's wedding earlier which had a completely different top part.

About the fake hair discussion, who cares. Many people have extensions or fake pieces put in when they go for certain hair dos. IMO, I don't think she had any in, Mary has thick hair. When one puts their hair up, the bottom back part is usually a slight shade different.